The present invention relates to a fixing apparatus that can be applied in an image forming apparatus such as a copying machine, a facsimile machine, or a printer, and also relates to an image forming apparatus including that fixing apparatus.
In an image forming apparatus of this type, for example when an electrophotographic method is adopted, an electrostatic latent image is formed on the surface of an image carrier such as a photosensitive drum. The electrostatic latent image on the surface of the image carrier is developed using developer, and a toner image is thereby formed on the surface of the image carrier. The toner image is transferred from the image carrier onto a recording sheet, and the recording sheet is heated and pressed to fix the toner image onto the recording sheet.
A fixing apparatus fixes a toner image on a recording sheet. That is, the fixing apparatus is normally provided with a pressing means for pressing a pressure roller toward a fixing roller. A recording sheet is transported while sandwiched in a nip region between a fixing roller and a pressure roller. Accordingly, the recording sheet is heated and pressed by the rollers, so that toner on the recording sheet is heated and melted so as to be fixed.
With such a conventional fixing apparatus, depending on the type of a recording sheet to be transported, pressure between the rollers may influence the state of a recording sheet when the sheet passes through a nip region. For example, when fixing is performed on a thick recording sheet formed pouch-like such as an envelope with pressure suitable for fixing on standard plain paper, a transport irregularity such as a wrinkle may occur.
In view of avoiding the occurrence of an irregularity in transporting a recording sheet such as an envelope as described above, a conventional fixing apparatus is provided with a pressure cancellation means that cancels pressure applied by a pressing means to a pressure roller against a fixing roller; thus, an appropriate pressure is obtained by the pressure cancellation means cancelling the pressure between the rollers as necessary (for example, see JP 2005-258135A).
On the other hand, there are also cases in which a fixing apparatus is provided with an external pressing means that presses a fixing roller from the outside. For example, in view of improving the heating capability for a fixing roller, such as shortening of a warm-up time or temperature following, an external heating means that heats and presses a fixing roller from the outside is provided as an external pressing means.
Generally, a fixing roller is a member to be replaced when maintenance is performed, and often attached to a fixing apparatus body that supports both ends of a fixing roller shaft so as to be easily replaceable. For example, there are cases in which both ends of the fixing roller shaft are positioned and held such that the fixing roller is movable to the pressure roller side (correctly positioned and held using an elastic member, such as a plate spring, such that the roller can be easily detached by a person such as an operator, for example), so that the fixing roller can be easily attached to or detached from a support member of the fixing apparatus body (for example, a member such as a lateral plate). By adopting such a configuration, it is possible to easily perform the work of replacing a fixing roller.
When a fixing apparatus including a pressing means that presses a pressure roller toward a fixing roller, a pressure cancellation means that cancels pressure applied by the pressing means to the pressure roller against the fixing roller, and an external pressing means that presses the fixing roller from the outside has a configuration in which the external pressing means presses the fixing roller from an opposite side to the pressure roller, and further the fixing roller is allowed to move to the pressure roller side, a problem as described below may occur.
That is, since the configuration allows the fixing roller to move toward the pressure roller side while the external pressing means presses the fixing roller from an opposite side to the pressure roller, when the pressure cancellation means cancels pressure applied by the pressure roller against the fixing roller, the fixing roller moves toward the pressure roller side due to a pressing force applied by the external pressing means. Consequently, the pressure between the fixing roller and the pressure roller that is expected to be appropriately cancelled is stronger than the pressure that should be cancelled, which causes an irregularity in transporting a recording sheet such as an envelope, for example.